One of the biggest problems with coming up with a module, or even a single encounter, is getting inspired. There are obvious sources of inspiration that a routinely mentioned; mostly movies and books, then there are others like old gaming books, other games, and history.
Movies and books are full of ideas if you look at them the right way; Indiana Jones style movies are full of cool skill challenges, just about any action movie has some sort of great set piece fight to draw inspiration from (tops of cliffs, on top of the church etc), and sometimes dramas, romances and comedies can give you ideas as well. The beauty of movies is they are so visceral that it is often relatively easy to compose the whole scene by watching it. The problem with movies and books is that game play often places very different demands on pacing and so on, so the mechanics can significantly alter the resulting experience.
Gaming products are hence often a much safer source of inspiration, but 4E products are often relatively sparse on material to be inspired by, which leads to problems in and of itself. I recomend a DDI subscription just to be able to get the ideas that flow through Dragon and Dungeon magazine, adding the ease of access to the crunchy stuff that the Compendium, Character Builder, and Monster Builder gives is just icing on the idea cake.
Now for 3E dnd one of the things you could do was browse the MM and read all that background material that is included with each monster, its tactics, the environment it prefers, and so on. Each of those things could tweak an idea for a module or an encounter. 4E did away with a lot of that stuff, but if you own the old books they are still great resources for getting those exact sorts of ideas and information from.
You can do similar things with other games products as well, Palladium RPG products are full of ideas that can easily be adopted for 4E DnD; their Cthulhu inspired Old Ones fit right into the Far Realms ideas of 4E. Warhammer FRP (and FB) have a nasty gritty setting that really suits the 4E “points of light” concept, and again Chaos = Far Realm provides plenty of fruitful ideas that can be adopted easily enough (take a look at the classic Shadows over Bogenhafen to see what I mean). Then there are the “old school” DnD products, many of the 1E and BECMI modules can be used easily in 4E – just be careful that you don’t fall entirely into Dungeon Crawl mode, one of the best parts of 4E is skill challenges, and plenty of things from those old modules work better as skill challenges than the fights they were originally.
Finally there is history; but the fact is a lot of “real” history is hard to use and use well, what is much better to go looking at is the “fantasy” history; the Illiad, and Anaeid. In addition to those “classical” sources there are more modern ones – Urban Myths. Urban Myths are a great source of not simply amusement but ideas, even if it turns out they were completly fabricated in the first place (Himuro Mansion), everything from the Crocodile/Alligator in the sewers to the Panther in the Forest, are things that PCs can get involved in finding out the truth about and perhaps end up fighting.
So next time you are looking for an idea, take a look in your books, or jump online and prowl through the urban legends and see what springs to mind.
(Now I just need to remember my own advice ;))