My experience with SiteBuiltIt was that it was great for people just getting started, but as an ongoing tool, it didn't really seem to provide a lot of value.
I use Market Samurai every time I'm doing any sort of niche research -- it covers so much ground...so much that you have to really filter through the info to get what you need. But it's only a one-time purchase price and then you're done; no ongoing monthly fees.
Some other niche discovery tools:
Google Trends tells you what's hot right now. Sometimes that can help you find a topic to cover.
Amazon and eBay both have top seller lists. You can see what's selling well and build a site or CPA offer from that.
CBEngine.com is a way to look at the hottest products to promote on ClickBank, which can sometimes give you niche ideas.
NicheProfitClassroom.com also has a great keyword tool -- one that Adam had built just for his members. It's a really cool, inclusive system for $67. One warning, though...the niche sites he suggests building aren't nearly as easy or lucrative as he makes them out to be. You can still make money doing it, but it doesn't come as fast as he says in most cases.
Of anything, I'd recommend Market Samurai. So many great applications for it.