Kayak - If you’re like me, and hate searching dozens of sites for the best travel deals then Kayak is what you’ve been looking for. With everyone being affected by high gas prices, searching for the best travel deals can go a long way in saving you money. Whenever I plan to travel, I first go to kayak.com to find the best deals. Kayak’s UI is clean, easy to understand, gives me the right level of filtering, and most importantly is comprehensive. It searches over 200 travel sites to give me the best deals on airfare, hotel, cars, and vacations. Only a few years old, Kayak is the clear leader in this vertical. It even bought out its main rival sidestep.com  for $200 million a few weeks ago.
Farecast.com - With a UI very similar to Kayak, Farecast was bought by Microsoft a few months ago. It’ll be interesting to see how it develops over the next few months and if Microsoft will be able to eat away at the travel search market. With Kayak’s success, I think that Microsoft should have never let Expedia go. Ideally, they should have kept Expedia in-house and placed its bets on making it the comprehensive travel search engine that Farecast aims to be.
Zvents  - When I met Zvents CEO Ethan Stock a few years back, I never thought I would end up using his service almost every weekend in the not so action packed Redwood City. Finding something to do in a small town is hard enough as is, and a search engine like Zvents goes a long way to solving that problem. Having tried out a number of other local event sites, Zvents by far has the best listings and UI.
Eventful  - I haven’t used this site as much as Zvents, but a number of my friends swear by it. Eventful relies on the community contributions which enables you to take advantage of some of the more random functions. While this may help you find a unique happening once in a while, I personally prefer a comprehensive and automated approach like Zvents.
Amazon  - Not many people realize this, but Amazon is steadily developing into a search engine for almost any product you’d want to buy. Today, when I shop on Amazon, half the products I get delivered are not even sold by Amazon, but by one of their trusted merchants. Hmmm…sounds like they are nicely going from just a commerce site to a complete search engine for products. Not a bad direction on their part. They already have the brand, they understand the unique UI needed for product search, and users trust them.
theFind.com - Siva Kumr, CEO of the thefind.com thinks that a pure focus on crawling commerce related products across the web and a great UI can make a big difference in product search. I tend to agree. His search engine is one that could give Amazon some heartburn in a few years.
Shopping.com - Owned by eBay, this is a big ad network as well as a money machine when it comes to the volume of transactions that happen on the site. My only concern is that eBay tends to be slow in innovating their properties and care too much about near term results (something all public companies have to do at some time or another). We’ll see if they can keep pace with companies like thefind.com and Amazon asI see a lot of competition in this space. Fortunately this can only be a good thing for consumers.
Spock - Yup. Not surprising that I would pick spock.com as the best people search  engine, as I may or may not function as the co-founder.. But I REALLY REALLY do believe that we have the best people search engine out there. Spock is the only people search site that seriously tries to index the web and create a real search engine. Compare results on Spock to any other site and let me know if you think someone else is better. I would love some competition in this space.
Seeqpod - A few of my engineers love to use this music search engine while coding away at Spock. It helps them not only find music they are looking for, but also has a discovery feature that lets you discover music that you may be into. It can be difficult enough to search or know what you are searching for, and a discovery UI is pretty helpful. However, don’t get too cozy with Seeqpod just yet. They are being sued by Warner Music for you guessed it (copyright). We’ll have to wait and see how this pans out.
Grooveshark - Similar to Seeqpod, they index web pages that contain user uploaded music files and have been catching up quickly to some established players in this space. We’ll have to wait and see what happens here as well with regard to some of the legal loopholes they may need to navigate through.
Truveo - It’s surprising to me how many people think that YouTube is the be all and end all of video. Sure Youtube may have 90% of the market share now, but so what! Netscape and Yahoo had that type of share at one point, and now look at what happened. In the world of the Internet, nothing is certain. That’s why I think video search is an attractive market on the cusp of something big. They do a really good job of indexing videos across the web. I was surprised at how well they indexed videos of me on the web.
Blinkx - It’s like watching 50 TV channels at once. A really unique UI and interesting take on video display. They are now even offering local video search - not bad. This site is definitely worth a peak.