This is definitely my favorite travel accessory, as one of the activities I most like to do on vacation is take loads of pictures. I actually have three cameras: one is a Canon Powershot SX130, a simple point and shoot that can sometimes take pretty impressive pictures. I enjoyed taking it to Europe, and it's a great camera to learn on because it allows you to get used to manual controls as well. Then I have a Nikon Coolpix S30, which was just a cheap $50 underwater camera that was a lot of fun to use at the beach or when it rained in Puerto Rico. Finally, I recently bought a bridge camera to use in Cuba. It's a Fujifilm FinePix S8200 that has 40x zoom and some more features like in-camera HDR and panorama shots, as well as manual controls. All of these cameras were under $200 and are my favorite travel accessories!
If you also like to take lots of photos on trips, Eneloop batteries will be your best friend. They last basically forever so you don't have to worry so much about constantly recharging batteries or missing shots!
Speaking of cameras and camera accessories, Gorillapods are really fun for doing some more advanced photography, like longer exposures for night photography or shots of larger groups (that include you!). Its small size makes it very portable, and it's so easy to wrap it around a railing or tree limb or set it up on a ledge and get awesome photos.
People all over the world know how to play chess, and this little magnetic set has come in handy a few times, whether it's passing time with people on train rides or in airports or making a rainy day go by in Bolivia. Other good options are a mini checkers set, dominoes or a deck of cards.
Something a lot of people forget about when planning a trip is power converters/adapters. A lot of hotel rooms in other places also have limited wall outlets, so it helps to bring a power strip in order to keep all of your electronics running.
I know that people like to use GPS's or Google Maps to navigate when traveling now, but sometimes you can't beat a good map. The internet can be unreliable in a lot of places and a GPS's directions can be inaccurate. Additionally, a map gives you a "bigger picture" view of where you're going, and you can check out things that interest you just a block or two away along your route.
As much as I love photography, it can't capture everything on trips. I like to bring along a notebook to write down interesting phrases I hear people use, directions or hours of different attractions, or foreign words or slang to remember. I also have a small sketchbook that I've enjoyed using when bored, or when you're at a location where pictures aren't allowed, as in many churches.
A cross-body purse is ideal for travel because it's cute, convenient, and minimizes the risk of pick-pocketing or bag-grabbing/slashing. It also makes it easy to take pictures and do things without having to balance a handbag on your shoulder.
Weather is unpredictable, and you can stay dry and keep your trip going (especially if you're somewhere tropical) with a 99 cent rain poncho. I also really like to bring an umbrella, not so much to keep me dry but so that I can continue to take pictures and keep my camera out of the rain.
On my trip to Bolivia last year, I finally broke down and bought one of these at the airport. I still have trouble sleeping on planes, but it did help me sleep on some bumpy bus rides in Bolivia. It's also nice in case you find the pillows at your accommodations a bit flat for your liking.
This list isn't an exhaustive one of all the things that are nice to have with when you're traveling, but it's a pretty good list of ones I actually use. What about you? Have you found anything particularly useful? What are your favorite travel accessories?