By means of comparison, a 7 oz cup of coffee has the following caffeine (mg) amounts, according to Bunker and McWilliams in J. Am. Diet. 74:28-32, 1979:

Drip                    115-175
Espresso                100mg of caffeine
1 serving (1.5-2oz)
Brewed                  80-135
Instant                 65-100
Decaf, brewed           3-4
Decaf, instant          2-3
Tea, iced (12 ozs.)     70
Tea, brewed, imported   60
Tea, brewed, U.S.       40
Tea, instant            30
Mate                    25-150mg

The variability in the amount of caffeine in a cup of coffee or tea is relatively large even if prepared by the same person using the same equipment and ingredients day after day.

Reference:
Variability in caffeine consumption from coffee and tea: Possible significance for epidemiological studies by B. Stavric, R. Klassen, B. Watkinson, K. Karpinski, R. Stapley, and P. Fried in "Foundations of Chemical Toxicology", Volume 26, number 2, pp. 111-118, 1988 and an easy to read overview, Looking for the Perfect Brew by S. Eisenberg, "Science News", Volume 133, April 16, 1988, pp. 252-253.

Quote from the lab manual:
"Caffeine is present in tea leaves and in coffee to the extent of about 4%. Tea also contains two other alkaloids, theobromine and theophylline. These last two relax the smooth muscles where caffeine stimulates the heart and respiratory systems."

The effects of theobromine are, compared to caffeine and theophylline, relatively moderate. However, cocoa contains eight times more theophylline than caffeine. As well, caffeine has been shown to combine with other substances for added potency. Thus the effects of theobromine might be enhanced by the caffeine in chocolate.

Theobromine is highly toxic to dogs and kills many canids/year via chocolate poisoning . It takes quite a dose to reach fatal levels (more than 200 mg/kg bodyweight) but some dogs have a bad habit of eating out of garbage cans and some owners have a bad habit of feeding dogs candy. A few oreos won't hurt a dog, but a pound of chocolate can do considerable damage.

Clinical signs of theobromine toxicity in canids usually manifest 8 hours after ingestion and can include: thirst, vomiting, diarrhea, urinary incontinence, nervousness, clonic muscle spasms, seizures and coma. Any dog thought to have ingested a large quantity of chocolate should be brought to an emergency clinic asap, where treatment usually includes the use of emetics and activated charcoal. The dog will thus need to be monitored to maintain proper fluid and electrolyte balance.

Pathogenesis of theobromine toxicity: evidently large quantities of theobromine have a diuretic effect, relax smooth muscles, and stimulate the heart and cns.

Reference:
Fraser, Clarence M., et al, eds. The Merck Veterinary Manual, 7th ed. Rahway, NJ: Merck & Co., Inc. 1991. pp. 1643-44.

On humans caffeine acts particularly on the brain and skeletal muscles while theophylline targets heart, bronchia, and kidneys.

Other data on caffeine:

Cup of coffee         90-150mg
Instant coffee        60-80mg
Tea                   30-70mg
Mate                  25-150mg
Cola                  30-45mg
Chocolate bar         30mg
Stay-awake pill       100mg
Vivarin               200mg
Cold relief tablet    30mg

The following information is from Bowes and Church's Food values of portions commonly used, by Anna De Planter Bowes. Lippincott, Phila. 1989. Pages 261-2: Caffeine.

Candy:
Chocolate                                      mg caffeine
baking choc, unsweetened, Bakers--1 oz        (28 g)  25
german sweet, Bakers -- 1 oz                  (28 g)  8
semi-sweet, Bakers -- 1 oz                    (28 g)  13
Choc chips
Bakers -- 1/4 cup                             (43 g)  13
german sweet, Bakers -- 1/4 cup               (43 g)  15
Chocolate bar, Cadbury  -- 1 oz               (28 g)  15
Chocolate milk  8oz                                   8
Desserts:
Jello Pudding Pops, Choc                      (47 g)  2
Choc mousse from Jell-O mix                   (95 g)  6
Jello choc fudge mousse                       (86 g)  12
Beverages:
3 heaping teaspoons of choc powder mix                8
2 tablespoons choc syrup                              5
1 envelope hot cocoa mix                              5
Dietary formulas ensure, choc,-- 8 oz         (259 g) 10
More stuff: Guarana "Magic Power" (quite common in Germany),
15 ml alcohol with 5g Guarana Seeds        250.0 mg
Guarana capsules with 500 mg G. seeds      25.0 mg / capsule
assuming 5% caffeine in seeds as stated in literature)

Guarana soda pop is ubiquitous in Brazil and often available at tropical groceries here. It's really tasty and packs a wallop. Guarana wakes you up like crazy, but it doesn't cause coffee jitters.

It is possible that in addition to caffeine, there is some other substance in guarana that also produces an effect, since it 'feels' different than coffee. Same goes for mate.