Aviation Specific

1st Spotters WWW
The only aviation enthusiast's site you'll ever really need (apart from this one, of course)! Absolutely phenomenal site, with a huge amount of information. Concentrates mainly on the civil airliner side of things, but the links section encompasses all aspects of aviation. Most useful section is probably the 'Spotter's Spot', with details about spotting possibilities at hundreds of airports around the world, sent in by numerous correspondants. Also plenty of news/photos of new colour schemes, photo galleries, thousands of links (including basic annotations, although there are now so many that a major reorganization would help to make them more easily useable), and all kinds of other goodies. If you're an aviation enthusiast, you need to check this out!

The 'AirNet' Links Site
A huge collection of links here, covering just about every aspect of aviation you could think of. Very comprehensively categorized, and in alphabetical order within each category, making it relatively easy to find what you want. No annotations though, so you've still got to visit each site yourself to see what's actually there, although giving that kind of information for everything here would be an all but impossible task, anyway! Updated regularly. If it's on the Web, it'll probably be here!

Airliners.net
An awesome collection of airliner photos here - I've no idea how many but it would take you weeks to look at them all! Basically a huge database of shots sent in by photographers around the world (you can do too if you don't want to/can't create your own website), and not just standard 'portrait'-type shots, either - there are also photos of airports, airliner flight decks, air-to-airs etc. All of this is fully searchable in any one of countless different search modes - outstanding!

Civil-Spotters Forum
A newsgroup-type mailing list, intended for civil aviation enthusiasts to exchange all kinds of information on their hobby. You have to be a member to participate, but that's just a simple matter of registering, and the list itself is very well organized and run. Occasional bickering does sometimes erupt, but in general the advantages far outweigh the disadvantages here.

Mil-Spotters Forum
Another mailing list, similar to Civil Spotters, but focussing on information related to military aviation.

Useful for the Aviation Enthusiast

Weather Underground
One of the many weather sites on the Web, this one could be particularly useful to enthusiasts since its data comes directly from weather stations at specific airports and airfields. Very comprehensive coverage from all around the world, although whether it's accurate is another matter entirely!

Mapquest
An excellent and extremely comprehensive map-finding site, which, although not always able to provide super detail, is more than good enough to give you an idea of where you want to go for almost any destination you can think of. It's also amazingly fast and simple to use, especially when you consider how much information they must hold and the fact that everything is necessarily graphics-based.

Universal Currency Converter
Very useful little gadget if you want to know how much local currency to take with you on your next overseas trip, or how much the entrance to the spectators gallery is in real money!

Tour Operators

Aeroprints
One of the longest running of the specialist aviation enthusiast tour companies, and with the biggest and most comprehensive Website. Updated fairly regularly.

Ian Allan Travel
A relatively new entrant in the aviation tour market, but part of the very long-running Ian Allan publishing company. The most professional-looking of the enthusiast tour Websites, but it would benefit from rather more frequent updating.

Pick Travel
Another long-running tour company, with a simple and would-be informative Website, except that it seems to be particularly out-of-date. Still, it's a starting point for getting in contact for further details.

Others

The Budget Traveller's Guide to Sleeping in Airports
Well, I guess that most of you reading this prefer a little more comfort on your trips, but if by some chance you do find yourself stuck at an airport overnight this could be useful! Even if you have absolutely no intention of making use of the information, you should still check it out - absolutely fascinating! Very comprehensive coverage from an amazing number of airports around the world, with a ranking for each. If nothing else, the report from Port Moresby, Papua New Guinea should make you think twice about going there for your next trip!!