Tour operators for adventure/challenge/trekking holidays?

[Roman]

Could anybody please tell me who the best tour operators/travel agents are for adventure/challenge/trekking type holidays?

Similar to the following:
Adventure marathons & triathlons
Del Sahara
Kilimanjaro
Machu Piccu
Race across America

I'm not looking for details about any specific event, rather which European tour operators specialise in such trips.

Many thanks

[Don]

I think these people are the standard by which others are judged in the UK.

http://www.explore.co.uk

The wife travelled with them a number of times in her single days...

[twinturboz]

Im heading out to Everest base camp on Friday with these guys. www.charitychallenge.co.uk. I can't really give you a proper recommendation on how the actual adventures are run as it's the first time I'm using them, but they seem to have good recommendations.

[itsnotarace]

http://www.jagged-globe.co.uk/

[Roman]

Thanks very much guys.

[Lanby]

twinturboz said:

Im heading out to Everest base camp on Friday with these guys. www.charitychallenge.co.uk. I can't really give you a proper recommendation on how the actual adventures are run as it's the first time I'm using them, but they seem to have good recommendations.

Hold on for your approach to Lukla

[neilski]

http://www.exodus.co.uk