http://www.io.com/~sjohn/plots.htm

Another source of plot twists is writers' prompts--they're meant to inspire ideas, after all. Again, print out lists and cut them into scraps, or open a book of prompts at random, or number them and roll dice. Here are a few online sources for writers' prompts (we also have a small handful in the writers' resources section of the Burning Void site); you can find more with a google search for "writer" and "prompt" (it also helps to add something like "spark," I found, to make the results more relevant):

Writer's Digest's daily prompt:
http://www.writersdigest.com/writingprompts.asp

Writing Fix's Random Daily Prompt Generator:
http://writingfix.com/dailypromptgenerator.htm

FictionAddiction.Net's daily writing prompt:
http://www.fictionaddiction.net/prompt.html

A Quick List To Get You Started

This is the quick list I put together for myself recently when I wanted it. It's skewed for a mission-oriented science-fiction game and for some of my plot preferences, but it's adaptable; change plot points and percentages as desired. It comes with numbers for a percentile (or d100) die-roll in case you want some randomly-gerated inspiration:

[1-2] The PCs' destination (planet, dimension, country, whatever) possesses exotic conditions that could make life complicated or interesting (exotic plant life, animal life, atmosphere, gravity, temperature, weather conditions, past man-made disasters such as extreme pollution or nuclear war, etc.).

[3-5] The population at the PCs' destination has been affected by unexpected conditions (religious wars, political upheaval, seclusion or segregation, invasion by outside forces, disease, starvation, etc.).

[6-15] Something happens to change the mission into a totally different type of mission. Choose a new plot. If you use a standard plot generation chart, use it again to determine the new mission. Otherwise, print out a plot list, cut it up or number it, and choose a plot at random.

[16-24] Something happens to add a new mission onto the characters' responsibilities. See the previous item for methods to determine this new mission.

[25-26] The enemy has fewer or greater resources than expected.

[27] A non-player character (NPC) expected to be absent is present, or vice versa.

[28-29] An unexpected sacrifice must be made in order for the mission to succeed, or for the characters to survive or return home.

[30-32] The characters find their way home barred, removed, destroyed or otherwise compromised.

[33-34] The characters lose communication with their forces, allies or commanders.

[35-43] The characters come across someone (individual, group or civilization) in need of rescue or aid.

[44] The characters stumble across a situation that is easy to misunderstand, and thus they may act inappropriately to the circumstances.

[45-47] The characters' headquarters, home base, families, or whatever are under attack, taken over or otherwise placed in danger.

[48] The characters' forces have been infiltrated.

[49-52] The characters are exposed to something dangerous--radiation, poison, disease, a drug, etc.

[53-57] The team is stalked or attacked (this could be the equivalent of a random encounter, or it could be a part of the plot).

[58] The characters stumble across unexpected guards or sentries and stand a chance of alerting them.

[59] A player character (PC) or major NPC ends up in a hallucinatory, hypnotic, feverish, or similarly altered state of mind.

[60-64] Equipment or information important to the mission is destroyed, stolen, or captured.

[65-68] A location important to the mission has been overrun or occupied by hostile forces or dangerous creatures.

[69] A natural cataclysm has changed the landscape and somehow interfered with the planned mission.

[70-74] Someone important to the mission dies, gets captured, or becomes injured.

[75-77] The team stumbles into someone else's problems and gets caught up in them.

[78-83] The team uncovers evidence of hostile activity beyond the scope of their original mission. (For additional complication: due to lack of communication, other events or time pressures they cannot call home to get new orders or aid and must act on their own.)

[84-85] Someone isn't who or what he says he is.

[86] An NPC disappears or leaves without explanation.

[87] Someone is being coerced or co-opted (blackmail, threats, seduction, mind-control, etc.).

[88] Exotic plot twist--a one-of-a-kind alien encounter, a major character develops amnesia, the team finds out they're trapped in a virtual reality device, a major character disappears or retires, or something of that level. This slot is reserved for those plot twists you can probably use only once.

[89] The team discovers that missing or presumed dead allies are being held captive.

[90-91] The characters were misled about details of the mission.

[92-93] A supposedly safe or innocuous location has been compromised. Its location is known, it's been bugged, there are troops waiting in ambush there, etc.

[94] The mission (or some portion thereof) is a ruse intended to capture the characters, get information from them, or something similar.

[95-96] Someone is attempting to use the characters for their own ends.

[97] Someone believed dead turns out to be alive. (This might deserve a second complication layered on top, such as that person having new allegiances, not remembering who they are, being captured by the enemy, having new resources if they're a villain, having been off doing something interesting, etc.)

[98] A natural disaster occurs, is about to occur, or has just occurred (volcano, flood, tornado, hurricane, earthquake...).

[99] Someone ambushes, betrays, or gives away the characters (intentionally or by accident).

[100] An enemy turns out to be an ally (or at least a potential partner), or vice versa, either temporarily due to specific circumstances or more permanently.