Depending on how serious you are about photography you kit can range from a simple compact camera or a smart phone to a full kit D-SLR with several lenses, filters etc.
I will assume that if you are reading this post is because you want to take more than just your smart phone or a compact camera. This is what I carry when travelling:
Main Camera D-SLR (Canon 5D MkIII)
Lenses:
Canon EF 28-300mm f/3.5-5.6 L IS USM - This is an awesome lens, when I first got it I thought that it would be the only lens I would ever need. Unfortunately after carrying it all day I found that it gets heavier by the hour and the telephoto end is rarely used when in a city environment. Nowadays I mostly use it for wildlife or surf photography.
Canon EF 24-105mm f/4L IS USM - This is my most used lens in built up environments, light and with a good range seems to fit most situations.
Canon EF 50mm f/1.4 - Great lens for low light conditions. Being small and very light I always carry it with me.
Filters:
A Fader Filter - This filter is very useful when one wants to produce long exposure shot in bright conditions.
Polariser - Mostly used for seascapes on very bright days
UV - All my lenses have a UV filter as a protection
Compact - Waterproof Cam (records 1080HD) (Lumix Panasonic 14Mp)
Memory Cards, Depending on the size of your cards and the resolution of your camera you may manage with just one card. But it is always advisable to carry a spare card just in case one gets trigger happy and fill your 32Gb card before you have the chance to get back to your hotel and download the photos.
A smart phone - I carry a Samsung Galaxy and its 8Mp camera has been very handy in more than one occasion. On another future post I will discuss how one can take great photos with a mobile phone.
A compact tripod.
Full size tripod - Even though one can always improvise using an stationery object and the camera bag to place the camera, for any long exposure or low light conditions it is very handy to have at least a compact tripod.
2 set of Batteries per device - No need to explain the reasons for backup batteries. Always carry a spare fully charged battery.
Chargers
Adaptors - if travelling overseas it is wise to get the type of adaptor required for the countries one is visiting.
Electrical Tape - Always useful to secure things, or to quick fix things like a bag strap.
Small Camera bag for day trips
Laptop - Ultrabooks are light and useful to have one to download you daily photographs at the end of the day.
External HDD - Copying and organising your photos at the end of the day make your life simpler when you are back home. Having an extra back-up provides an extra guarantee that no images would be lost if something goes wrong with the laptop.