From Europe there are several routings via the Middle East and many of the Middle Eastern airlines: Emirates, Qatar and Gulf offer daily flights to Kathmandu with a stop-over at their respective hub. You do need to be careful looking at these flights though as some have very long layovers which can turn a very long trip into an excruciatingly long one. Our personal preference is Qatar who offers great service and seems to manage incredibly short layovers in Doha.
The other alternative is to fly with BA, Air India or Jet to Delhi and then catch a shorter flight up to Kathmandu. Again be careful of long layovers and be warned – some of the reviews for Air India are less than glowing!
Similar options apply to travellers from North America where again the Middle Eastern airlines offer the best options.
Expedia, Ebookers are all good for the long haul flights but Skyscanner seems to be the best and the one we recommend at the moment.