Recently I read a meta-analysis of what works and what doesn't for a common cold. You can find it here.

Some highlights from that article:

Vitamin C
Prevention
Generally does nothing if taken as a prophylactic, but...
"prophylactic vitamin C (when given to marathon runners, skiers, or soldiers in subarctic climates) reduced the incidence of the common cold by 50%. This appears to suggest that, when taken during periods of stress when the body's immune system may be weakened, vitamin C may help to prevent the onset of the common cold." and
"In addition, among individuals who experience a cold when taking vitamin C preventively, the duration of the cold may be reduced by 8%; the impact on the severity of the cold may only be slight, however"
Take home message - it probably makes sense to take vitamin C during the times of stress to prevent cold. If you do take vit C chronically your colds will be shorter by a whopping 8%!

Treatment
"one study did find that a single 8-g dose of vitamin C, when taken at the initial onset of symptoms, reduced the duration of the cold to 2.86 days, compared with 3.52 days with placebo. Vitamin C treatment does not appear to reduce the severity of the common cold, despite one study that found a slight benefit to its use"
Take home message - 8 grams of vitamin C in any form (even the "good" Ester-C kind) will give you epic diarrhea, but it may also cut the duration of your cold!

Echinacea
Treatment and prevention
"echinacea versus placebo for prevention, no statistically significant difference was found in terms of the number of colds, their severity, or duration"
"Two studies evaluated the efficacy of echinacea versus placebo in individuals who developed a complete cold on the duration of the cold. Whereas one study showed statistical significance, the other did not."
Mixed results in both treatment and prevention - it might or might not work

Zinc Gluconate
Lozenges
"The author of this review concluded that fair evidence suggests that zinc gluconate lozenges reduce the severity and duration of the cold when taking at least 13.3 mg of elemental zinc every 2 hours while awake starting within 24 to 48 hours of symptom onset"
Intranasal
"zincum gluconicum nasal gel spray shortened cold duration and reduced the severity of symptoms if initiated within 24 to 48 hours of symptom onset."

Although some studies have shown zinc lozenges to not be effective - there is significant evidence that they do work. Nasal gel seems to work the best