Joined TR when I googled "Morrowind Mainland". Because I knew that's all I cared about. Joined the project in February 2005 when I was thirteen (I'm 22 now) Those days, every other day at TR was pretty much like this.
(myself included)
Good times.
"Idleness and lack of occupation tend - nay are dragged - towards evil."
-Hippocrates
Come to think of it, I probably came across Tamriel Rebuilt when browsing [url=http://www.mwmythicmods.com/]this webpage[/url], back in my early days learning about Morrowind mods.
-Head of NPCs: [url=http://www.shotn.com/forums/]Skyrim: Home of the Nords[/url]
The UESP. Ironically enough, being present on the Unofficial Elder Scrolls Pages gave TR an air of officialhood to me.
'The strange thing about TR is that I think it is by and large accepted that we will finish. We are all the sort of crazy people that would do such a thing. We are inevitable.' ~ Thrignar Fraxix
I found out about TR through Planet Elder Scrolls while looking through their landmass mods, and was so excited to see that people were making the mainland.
If I recall, it was through the Elder Scrolls threads on 4chan's /vg/ board. A lot of people over there have been playing through the updates regularly.