the teach yourself gaelic book is pretty good.
some free stuff:
http://www.taic.btinternet.co.uk/
http://www.bbc.co.uk/scotland/alba/foghlam/learngaelic/index.shtml
http://www.smo.uhi.ac.uk/gd/cursaichean/cg/index.php
if you can afford it, take a short course at sabhal mòr (the last link).
this site is really good for explaining minor details, not much good when you're starting off, but worth looking at and remembering
http://www.akerbeltz.org/beagangaidhlig/gramar/beagangramair.htm
free dictionary
http://www.smo.uhi.ac.uk/gaidhlig/faclair/sbg/lorg.php
write the english word in the box and press LORG (or just enter). change the scroll down from BEURLA (english) to GÀIDHLIG (gaelic) depending on what language the word you are inputing is. most of the other boxes are useless.
the gaelic section on unilang has people who are willing to help you with problems. so does fòram na gàidhlig. if you speak german, the forum at
www.schottisch-gaelisch.de is, strangely, probably the best.