Isometric arm exercises are one of my favorite topics to cover. There is something that draws immense attention to the arms and their development that really goes beyond just the visual. For day laborers like masons, smiths, and forgers, arm strength was always critical, and having strong arms was akin to being able to doing more work.
Arms are also a easy way of measuring machismo, with the age old arm wrestling competition crossing cultural lines as a rating of strength and masculinity. If you wanted to convey the strength of a fictitious character, you drew them with powerful arm muscle and definition.
Look at Superman and Batman over the years; their designs have allowed for larger, stronger looking arms each generation. Naturally, this huge multicultural emphasis on arm strength motivates people to train their arms like crazy.
Handstand pushups (one of my favorites) are great arm exercises without weights, and really call on full body strength and balance to complete. Snatches are great free weight arm exercises that aren't seen too often in conventional gyms, or taught by many personal trainers.
As demonstrated by the incredibly powerful physical trainer, Bud Jeffries, doing both will really give you some true power, instead of giving you weak, swollen arms.
The list of methods to train your arms goes on and on...so naturally, I will add to that ever-growing list that goes far beyond just curling. Except these examples will be simple, effective at all strength levels, and time-friendly. As time friendly as, say 7-12 seconds.
First of all, I ask that you define the kind of muscle you want to see on your arms. High volume training is great for getting your arms to pump up over time, but that muscle growth is mainly sarcoplasmic (extra glycogen stores) instead of dense, myofibrillar growth. The latter would be more along the lines of Bruce Lee's sinewy strength, but as fit as Bruce Lee was, I've rarely heard people aspire to his arm size.
If you want arms that look powerful and feel powerful, you need isometric exercises that will give you both sarcoplasmic and myofibrillar growth.
Try this combo of exercises...before I proceed, note that myofibrillar specific exercises require a longer recovery time than sarcoplasmic specific exercises. While the 60% intensity exercise can be performed roughly every three days, the 100% intensity exercise should have at least a week interval before training again, or you risk under-recovery.
Stack both fists on top of each other. Now apply pressure with both hands, attempting to bring one fist down while trying to bring the other fist up. Do this with as much intensity as you can muster, I mean arms shaking and burning, for 7-12 seconds.
Once you've done that, take a few rest breaths, and stack both arms again. Now do the same thing, with 60% of the intensity you used before. You'll still do the exercise for 7-12 seconds, but repeat that four times.
To keep that clear, 100% intensity for one set of 7-12 seconds, and then 60% intensity for three sets of 7-12 seconds. Going at 100% intensity with rapidly fatigue your muscle and strengthen your tendons, allowing that dense, myofibrillar growth.
Going at 60% intensity for reps following that will further fatigue your nervous system, and nerve fatigue (and proper rest and recovery) is what makes the body send out the hormones needed to add the mass that your arms have always desired. Try it out and let me know how it works for you.
Please put any thoughts, questions, or feedback here and I'll respond as soon as I can.