General information:

Yamada Language Center Home Page
I Love Languages
Omniglot -- a guide to writing systems
Language Varieties
KryssTal's Language Page

Learning languages:

Foreign Languages for Travelers
Your Dictionary
Languages on the Web

Interesting languages:

European minority languages
Lingua Franca
Milanese
Catalan
Irish
Breton
Gothic
Basque
Basque
Quechua
Quechua

Artificial languages link sites:

Constructed Languages List
International Auxiliary Languages
Constructed Human Languages
Model Language Index

Specific artificial languages:

Tavo (minimalist Lingua Franca Nova)
Interlingua
Ido
Latino sine Flexione
Slovio (based on Slavic languages)
Proslava (a better panslavic, by George Doudy)
Nordien (pan-Germanic)
Brithenig (if Latin had lasted in Britain)

Spelling reform:

Spelling reform
Spelling reform
Simplified Spelling Society

And just for fun:

Numbers in over 4000 languages