Being able to track topics on the internet can be pretty tricky as there is so much out there. So what I have below is a list of tools which you can use that use both feed, email and services to deliver the information you need

Feed and Email

Google Alerts
PubSub
Yahoo Pipes
Topikality
Swammii ( No RSS )
Woofeed ( No RSS )

Destination Services

LazyFeed
Regator
My6sense
Technorati
Ensembli
Guzzle.it
Cascaad

Social Options

Twitter Search
Delicious Feeds

Community Based

Digg
Reddit
Hacker News
SlashDot

People Created

Tumblr
Posterous
Favit

Aggregators/Portals

Tweetmeme
Equentia
Alltop
GoogleFast-Flip Feeds

Misc

Google Trends
Mash Logic