Everyone always seesm to laugh at Yamcha and treat him as weak and a bit of a joke. I personally think this may be a bit unfair, whilst he is obviously never in the same class as the Saiyans or Piccolo I have him at a very similar standard to Tien and Krillin. Here's why...
At the outset of the Saiyan saga it is stated whilst he and Tien are sparring as part of their training that they are just about completely evenly matched
Yamcha like Tien makes mincemeat of his Saibaman and only dies through his cockiness in assuming it dead and turning his back on it.
He recieved the same training at King Kai's as Tien and Piccolo and was actually the first (between himself Tien and Chaoutzu) to catch Bubbles and hit Jeremy.
He is consistently shown to be relatively realistic in regards to his own power and his limitations and still showed up to fight the androids after the three year training period, even knowing they would be stronger than Freiza. This sugests his own power developed substantially.
He was able to last, albeit lose in a fight against the Cell Juniors which were a challenge for even Vegeta and Trunks.
It's obvious that he was never in the same ballpark as the likes of Goku, Gohan, Vegeta, Trunks, Goten, any of the main bad guys in their own time frame or Piccolo but neither were Krillen, Tien or any other human. People often comment that yamcha never won a fight throughout the whole of Z, yet Tien or Krillen did not win one either unless you count the Saibamen or some of Frieza's no-name goons.
What I'm saying then is that Yamcha is right up there alongside those two vying for the strongest human slot. While he may just miss out (I personally think it's Krillen), he's far from insignficant. By the end of the series his own, Tien and Krillen's power levels although not known exactly are approximated at anything from 500,000 to 5 million, with some debate even as to whether they could defeat Frieza. While they pale in comparison to some of their ridiculously strong contemparies they still possess a remarkable power which would have been thought unreachable at the outset of Z. Give the guys especially Yamcha some more credit! :P