Everyone personal or business should be using search.twitter.com. Think about periphery keywords (curiosity keywords), locations, emotions, etc (see advanced options) that may not directly create "marks" for you to engage with but more pull you into conversations that build your knowledge of landscape and the people moving with in it.
Of course there are plenty more which I look forward to seeing on this page.
I use www.samepoint.com and like it.
According to them: "What we look at"
As social media has taken on many different forms, we've expanded our crawlers to encompass every conceivable type of social media service. Our categorization breaks out results by type of social media, scouring the following:
* micro-blogging platforms like Twitter Identica
* video platforms such as YouTube and DailyMotion
* social bookmarking communities such as Digg and Delicious
* blogs and associated comments
* social networks like Facebook, MySpace and Hi-5
* B2B Networks such as LinkedIn and Plaxo
* Groups like Yahoo Groups and Google Groups
* Q&A sites, such as Yahoo Answers and Yedda
* Life Casting sites, such as DaveTV
* Review sites, such as Yelp and CitySearch
* Web Sites