The TAC is a "competition" or "learn-a-long" at the HTLAL-forum, usually brought about by Reineke and with many happy participants. There have been two TAC's so far, and the third one is to begin for 2009 and last one year. The rules for the TAC vary somewhat from time to time, but they involve setting goals for your learning in one or more languages, keeping a log in the HTLAL log section or elsewhere and trying to get as far as possible. The one who by his or her logging shows the most progress at the end of the competition is the winner, and the rest of the participants are annihilated.
These are excerpts from the original thread at the HTLAL forum by Reineke concerning the 2009 TAC:
"At the very start please state your current knowledge of your target language(s). Work out a plan or simply start without one. Do declare your target languages and whether or not you intend to follow a particular method. Maybe you CAN do Assimil every day.
You may start a new log or rename an old log. Add TAC/09 to the name. In case you’re reusing an old log edit the first post to account for this. Mention the page number where your 2009 log entries begin. It is ok to keep a record somewhere else and link to it. Create log entries, blog posts etc. and try studying as much as possible – certainly more than you would normally do. Write about course materials, hours studied, and document any major effort and this includes things like TV, movies, comics and music. It’s ok to give weekly, biweekly, or even (hopefully detailed) monthly reports. Log entries should not be a chore.
The ultimate goal is to have fun, remain motivated and actually accomplish something concrete. As the very minimum for 2009 you should bring two languages to a useable level."
"For your other languages, those that qualify as "intermediate" or "advanced" you should be able to declare a noticeable improvement and claim the next proficiency level. For a language that is very advanced but neglected, you should be able to report that tingly sensation that your favorite language has been revived and is alive and kicking. For anything deemed advanced you should present a sizeable reading list."
There is also a thread with tips on how to write your TAC log and make it interesting.