For many years when I lived in Santa Clara, CA my favorite niche in ham radio was QRP, with its huge variety of kits. Some advantages of this niche are: (1) It is very affordable for buying a new rig (often under $100), (2) building kits is fun, (3) making CW contacts with minimal power really improves one's CW skills and (4) there is a large QRP community of skilled and helpful QRPers to help you along the way. I was fortunate to live in Silicon Valley during the height of the Golden Age of the Norcal QRP Club, which sold a great series of kits all over the world, had its own printed publication, as well as monthly meetings in a restaurant near the monthly flea market in the East Bay of the SF Bay Area. Norcal eventually ended, largely brought about by the death of one of its two founders, but Elecraft became a very successful commercial successor to Norcal and there are still many other active QRP clubs all over the world providing lots of great resources and fun activities.  

I stumbled upon a compilation of QRP kits posted by Neil W2NDG in April 2012. It was pretty comprehensive at the time, but of course it is considerably out of date now. But I decided to link to it anyway, as it might still be useful to some just starting out and as a bit of nostalgia for the old timers. An updated version would be a very worthy project for someone wanting to contribute to the current QRP community.   

Some updates about some of these resources are provided below.  

• Doug Hendricks, the surviving founder of Norcal, developed his own line of kits, as shown by W2NDG, but then sold them to James Bennett who had developed his excellent Pacific Antennas, which he and his wife still offer along with the Hendricks kits and many more new kits in an ever expanding product line.
• Hendricks went on to work with some other folks at The QRP Guys, specializing in very inexpensive kits.
• Ten Tec seems to have become mostly dormant, though its popular support mailing list is still active.
• Dave Benson retired his super successful Small Wonder Labs, built a mountain retreat, sometimes offering kits through the Four State QRP Group and more recently teamed up with George Heron to produce a new line of low power digital rigs called Phasers, a very welcome return of a very talented designer.
• Wilderness Radio was retired after many years of providing a very successful line of rigs designed by N6KR, one of the founders of Elecraft. Their radios still appear in the used market and are a great addition to any QRPers shack. 
• Four State QRP Group has continued to produce an excellent line of QRP gear, still highly recommended.
• Marshall Emm, who owned the excellent line of OHR and Ameco gear passed away and it is not yet clear what will happen with his popular Morse Express website, which seems to be on a temporary hold.