Dangerzone
thats the tutorial that helped me best getting started with radiant. go there and click q3radiant tutors to grab a german tutorial zip.

Bubba
great tuts by bubba, good tips for beginners as well as for experienced mappers.

Quake3world
having questions? the best forum to get help on q3 mapping.

Quake3.qeradiant
huge list with links to tutorials.

Q3W-thread
thread at quake3world bout mapping tricks, nice1.

Fragpoint
very usefull Wolfenstein tutorials (german).

Nightwulf
best tutorials about RtcW scripting (german).

Tramdesign
nice site with a huge RtcW forum.

Kos
Kos forums, dedicated to RtcW Sp-mapping, dont miss it.