There are many problem chess sites around where you can find extensive link lists. The following list is a pick of my favourites:
Links to other problem chess sites
Commission for Chess Composition - Official site of the PCCC (Permanet Commission of the FIDE for Chess Composition); grab the latest news here concerning FIDE-Albums, World Solving Championships and titles!
Mat Plus Home Page - Big site by the main Yugoslav magazine; many articles and tourney awards.
Ralf Krätschmers Homepage - A site run by a German fellow composer. Mainly moremover orientated.
Torsten Linß' problem chess pages - Another successful German composer, FIDE master, presenting his best problems (orthodox and fairy).
Glanz auf 64 Feldern - Problemkunst in Oberösterreich - Selected problems by the Austrians Sladek, Gamnitzer, Schmoll, Fölser, the first two being top international composers.
Stanislav Vokal's Chess Compositions - "Best of"-collection by a versatile Slovak composer.
Juraj Lörinc's Chess Composition Microweb - Big site by the Slovak Fairy Chess composer; informative and useful.
Alexander Zideks Schachprobleme - Don't be irritated by the rather basic layout and the pop-ups, the quality of Alexander's problems is very good.
Brian Stephenson's Web Site - "At this website you can find out about the British Chess Solving Championship, the British Chess Problem Society and chess problems in general." Interesting site by the English composer and solving official. You might as well have a look at Brian's blog: http://bdslog.blogspot.com/
Schach - Problem pages of the renowned German Schach magazine run by the prominent composer Udo Degener.
L'Italia Scacchistica - The main Italian chess magazine with a good problem page.
Chess Composition in Torun - Especially the selected problems by Waldemar Tura and Marek Kwiatkowski, two outstanding Polish composers, make this site worth a visit.
Narayanan's Problem Corner - Many basic articles by the leading Indian composer.
The Danish Chess Problem Society - interesting site with many articles, portraits of Danish composers, news about tourneys, a gallery a.s.o.; mainly in Danish though, but you'll find some English pages.
Die Schwalbe - The site of the flagship of German chess problem magazines; a very useful feature is the latest INFOBLATT with extensive information about forthcoming tourneys (now up again!).
Problemschach in Österreich - Problem pages run by four computer science students of the University of Graz and hosted by the Austrian Chess Federation: Chess Problem of the month, database with selected problems (you can even upload your favorites there!), a problem chess dictionary and links (in German).
Problemesis - "French-English bilingual web-magazine devoted to chess problems" hosted by the French composer Christian Poisson. Up-to-date tourney reports, a comprehensive list of problem theme definitions and much more.
PDB Server - Chess Problem Database Server. Extensive problem database, search for authors, sources, stipulations, themes and so forth (Hey, they even host some of my problems - one other reason why I have to recommend this site ;-)
Links to Macintosh Chess downloads
Popeye 3.21 (Macintosh version) - The only sensible solving program for the Mac; a bit slow if you try to solve longer problems (okay, the Mac G5 Dual 2,7 GHz should cope with a #10 in seconds: anybody buy me one? :-), but very comfortable for solving 2-5#s. Many fairy chess options; languages: English, German, French.
TOPeye (Macintosh version) - Popeye's graphic interface for diagram input (in German, though).
Vektor3 Chess - Another classy playing program for the Mac; give it a try: it's so well designed (Shareware 19$).