You probably don't know the rule in buying lenses yet. As much as possible, get the same brand as your camera. If you can not afford the original brand, only then do you get the equivalent or approximate equivalent from another brand. If you do that, make sure that you are getting the lens with the same mount as your camera. Just don't complain about certain compatibility issues. If you didn't get the original brand, it won't be 100% compatible.
The most common third-party brands are Sigma and Tamron. Sigma is generally better than Tamron. Among the third-party brands, Tokina is undoubtedly the best.
There are many others like Vivitar, and Promaster. Most are cheaper versions of the original.
To a select few photographers, the original brand won't be enough. That would be the time to look at the higher quality lenses that have greater tradition and pride behind their names than the more popular brands. Some of these are from Zeiss, Voigtlander, and Schneider. Of course you can't blame those who realy know it for spending a fortune just to be able to adapt a Leica rangefinder lens to his dSLR.
Now you know!