Lots of great answers here!
I think one of the most effective methods is the ascending pitch, as mentioned by several others. the reason this works so well, is that it targets a very primal reaction in us. Humans naturally/instictively try to sync ourselves with our surroundings, and this can bee seen in almost every area of our lives. We cry when others do, we laugh when others are, we get caught up in the moment when in a crowd/group environment (mobs, concerts, etc.), getting irrationally freaked out when the rest of the group does, sync-ed periods in a women's dorm room, your heartbeat trying to sync with the rhythm of a drumkit, etc. In light of this, look at how humans express intensity. An increase of intensity in an emotion, is directly matched by an increase in pitch. If I win the lottery, the pitch of my voice goes up the happier I get. If you wreck my new sportscar, the pitch goes up the angrier I get. The pitch of screams on a rollercoaster almost directly mirror the intensity of that particular turn or drop. The harder you twist your ankle, the higher you exclamation will be. When you step on a weak board, the pitch of the creak rises the more strain you place on it. Strain always elicits a higher pitch - especially when approaching the breaking point. An animals cry of alarm is quite often the highest pitched sound it makes.
While useful for escalating any type of response from your audience, it indeed works quite well for tension. For instance, consider the scenario given earlier about the climber's rope fraying. Try taking those sfx, and slowly pitching them up, over time. The tension in your audience will rise right along with it. Imagine the hero approaching a door to open it... but unseen behind this door lurks the monster, ready to leap out. Your audience is already tense, having been cursed with the knowledge of his impending doom, but now slowly begin to pitch his footsteps up, and their tension will rise to new heights - right along with the feet. Each ring of an urgent phone call, is subtly higher than the last... Rinse and repeat.
Another area with incredible potential for creating tension can be found here.
The heroine is hiding in the closet, as the experimental creature is ransacking the lab. Suddenly she (along with your audience) is overcome with utter dread, as the room grows deathly silent. No longer able to auditorily track the creature's whereabouts, one can only tremble in silence - awaiting the moment the door is suddenly torn off it's hinges. Or consider the red flags that appear, when a parent realizes that their child in the other room has gone strangely silent. This means Junior is into mischief again...