If anyone’s feeling depressed about the future of Silicon Valley innovation, I’d suggest sneaking into the demo day tomorrow at incubator Y Combinator‘s Mountain View, Calif. office. Watching a bunch of smart, ambitious entrepreneurs show off what they’re working on is a good cure for the “nothing cool is happening” blues — and I saw a strong batch of startups at today’s event. (The same presentations repeat tomorrow.)

For the most part, there weren’t many “wow” moments or big conceptual breakthroughs, but I saw some cool ideas, promising products, and plausible business plans. Virtually all of the companies were web startups, but relatively few of them seemed like “me too” ideas. And even when they were, their boast that they were “like Famous Product X, but done right” actually held some water.

So here are my five favorites among the companies I’m allowed to talk about, ranked in order of how promising and interesting I find them, followed by short descriptions of the other presenters. Some of the demonstrating companies — including my favorite — are still in stealth mode, so I can’t talk about them yet.

1. Cloudkick — If you want to run your web applications using online infrastructure built by Amazon, Google, Microsoft, or any of the other Internet “cloud” providers, Cloudkick says it has the best tool to manage everything. The closest competitor is probably RightScale, but co-founder Alex Polvi says Cloudkick offers the easiest interface out there (judging from the demo, it looks more like a consumer application, rather than a more complicated enterprise app), and is also dedicated to managing applications across multiple cloud platforms. The company launched its product on Monday, and it’s already managing applications across 1,600 servers. Among its customers are 40 Y Combinator startups — and while YC companies often support each other, Polvi notes that these companies are basically trusting Cloudkick with their business. That isn’t something you do out of loyalty alone.

2. Wattvision — These guys probably had the coolest presentation. Beforehand, they attached one of their sensors to the office power meter. Then they opened a web page showing a chart of the office’s electricity usage; when they turned off the lights, the chart reflected the reduced usage a few seconds later. It can’t tell you exactly what’s causing the usage, but since the data is updated in real-time, you could figure it out by just walking around the house, unplugging devices, and seeing how your power usage is affected. There are other companies looking to accomplish the same goal, most prominently Google with its PowerMeter, but Wattvision says it’s the only one whose devices work with the existing power meters. That means it doesn’t need to wait for regulatory breakthroughs or technological advances.

3. Heyzap — The company describes itself as the YouTube of Flash games. Basically, its technology makes it easy to distribute a developer’s casual video games throughout the web in the form of an embeddable widget that can be placed on media portals, site creators, social platforms, blogs and individual sites. If a developer is trying to make money through ads (either of its own making, or from Heyzap), then the more eyeballs the better. Heyzap released some impressive usage numbers last month.

4. thesixtyone — A site that uses a Digg-like interface to let users find, listen to, and vote for their favorites among songs uploaded by independent musicians. It sounds like these guys are doing a bunch of things right, such as building their own infrastructure for cheap web streaming, allowing users to keep playing a song as they explore different parts of the site, and encouraging users to choose good music with achievement badges and other rewards. I wasn’t really sure that the world needs another music site, but thesixtyone already has some impressive stats — it says it has more than 100,000 unique monthly visitors, and even better, the average visitor session lasts 40 minutes.

5. Propable — OK, this one’s a bit different. While Propable’s founders say the key to their success is software, they’re actually in the property management business. The basic concept is to rent apartments, then divide them into individual rooms. Those rooms are then rented out, with furniture and service, to individuals. This should allow young, single professionals, especially those who spend very little time at home, to find housing that’s either more affordable or higher quality than most studio apartments. The company says it’s already operating more than 40 units in London, and that aforementioned software will allow the operation to scale far beyond that.

And here are the other companies, in alphabetical order:

Divvyshot — Simple group photo sharing. Divvyshot is particularly promising (from both a usability and business perspective) in how it can pool photos from a single event, like a wedding. All attendees can email their photos to a single address, then browse everyone’s pictures in a single group.

Echodio — Synchronizes your iTunes tracks across multiple computers. The company also presented a bigger vision of enabling other kinds of cloud music services.

Foodoro — Online marketplace for specialty, artisanal foods. Also allows the food-makers to promote their wares through embeddable widgets.

nambii — Developing a range of paid mobile dating apps, with an emphasis on fun, simplicity, and tailoring the experience to the iPhone.

Skysheet — “Spreadsheets done right.” Basically an online spredsheet program with features such as hyperlinked spreadsheets, spreadsheets in cells, and functions without programming. Several other demo day attendees seemed really excited about this one.

Voxli — Creates voice chat rooms with just one click. The initial audience is online gamers, but some users are treating it as an alternative to Internet phone service Skype.

Incidentally, Y Combinator just announced a $2 million investment from Sequoia Capital, which should allow it to invest in more companies.