Twitter Memes and Hashtags:

Follow Friday
Each Friday, Twitter users suggests other Tweeters to follow. They end or begin their Tweet with #FollowFriday (An example.)

Gr8t Tweets for the month of March
Re-Tweet (RT) one great Tweet a day and include the hashtag #gr8t at the end.  All Gr8Tweets show up on the Grt8Tweets Wiki home page.
Here’s a list of who’s participating. (Though I’m sure there are many, many more.)

Using Twitter Hashtags – This is a great explanation of what a Hashtag is and how they are used.

Ways to Build your Network:

Twitter4Teachers – An extensive list of educators on Twitter  – categorized by subject area / grade level
Who Should I Follow? – Find new Twitter Friends
Mr.Tweet – Discover great people relevant to your current needs

Searching for Tweets and Twitterers:
Twitter Search – Search by keyword, Hashtag or even Twitter ID
TweetScan – Searches Twitter and allows you to get e-mail updates
Tweetdeck – Group people together and have separate columns for @Replies, DMs, Groups and the public timeline

Cool Twitter Tools:

Tweet Wheel- allows you to visually discover which of your followers know each other.
Top Twitter Friends -  Including a list of your top 20 BFFs and suggestions of Twitterers to follow.

For discovering many more Twitter tools:

Top Twitter Tools for 2009
Twitter Fan Wiki

I know there are so many more tools and resources out there. Many folks in my PLN have created screencasts and video tutorials on how to use these tools and on the merits of using Twitter. They are all in my Social Bookmarks at http://delicious.com/lthumann/twitter or http://www.diigo.com/user/lthumann/twitter. I also bookmark anything related to Twitter to the Diigo Twitter Freaks Group. Please join us!