Twitter has a rather well-buried feature that lets you control the retweets that you see in your twitterstream.
As far as I know this can only be done through the GUI, I don't see it listed as a Twitter command.
You can disable retweets per-user, that is, if you want to see text directly entered by someone, but not anything they just retweet, you can block seeing their retweets.
Login and go to the user profile (you can only do it per-user, not globally).
UPDATE 2012-02-20: How to do it in the updated Twitter interface...
On the profile page for a user, click the "head" icon (next to the Following button). Select menu item "Turn off Retweets" (indicated with red arrow).
For the old Twitter interface:
Find the small circle with "retweet cycle" arrows and click it (indicated with red arrow in image below). Retweets done by this user will no longer show up in your timeline. In the example below, if you clicked the circle, anything retweeted BY @scilib (me) wouldn't show up in your timeline any more.
There are also some search commands you can use with retweets. Unfortunately there is almost no documentation. Some appear to apply only to RT style retweets, some to both Twitter retweets and RTs.
You can try these parameters in a twitter search:
filter:retweets (show ONLY retweets)
-filter:retweets (everything EXCEPT retweets)
exclude:retweets (everything EXCEPT retweets)
include:retweets (include retweets)