Like Cord says, isometric exercises shouldn't be your primary or only form of strength training, but they are great as a supplement to your other strength training. They can be done anywhere, a workout can take as little as one minute, and their efficiency at building strength is well documented.
Isometric training has been shown to use a greater fraction of a muscle's fibres, which translates to higher tension and greater strength gains. Isometrics are ideal if it's 'wiry strength' you're after, especially if you keep the contractions under ten seconds. Not to sound like one of those Bruce Lee fanboys who invariably crop up on forums like this one, but it's interesting to note that Bruce Lee used isometrics a lot.
Unlike strength training with weights, you can and should do isometrics every day.
There are different schools of thought on how long to hold the tension, how much tension to apply, and how many sets and reps to do. Personally, I use 4 reps of 6 second maximal contractions. Check out the links below and find what works for you.
A few caveats:
Isometric strength training builds strength specific to the angle the joint is trained at, although 50% of the strength gains carry over to twenty degrees either side of the angle trained. This means you have to train at a variety of angles.
Strength gains from isometrics will stagnate after 6-8 weeks, so make sure you're using dynamic strength training as well.
Isometrics can reduce flexibility if applied at positions well below full extension. Make sure to stretch the muscles you've just worked.
Here are some exercises:
Stand in a doorway. Place both hands on the lintel. (Is that the word? I mean the top of the doorframe.) Push up hard , not just with your arms, but with your whole body: legs, abdomen, chest. Vary the angles - do it with hands together, hands apart, feet together, feet apart. Also, try it while standing on your toes.
Grip the top of the doorframe and pull down, again trying to recruit as many different muscle groups as possible.
Grab an immovable bar low down and try to deadlift it, i.e. drive your legs into the ground and pull up. (I use a radiator pipe for this one.)
Try squats at various depths - with thighs parallel to the floor, with thighs at forty-five degrees, etc. (It can be difficult to find something appropriate to push against, but you can use a doorway again if you extend your arms fully overhead.)
Put your forearms under a desk and push up hard.
Place your hands on a wall with your arms fully extended. Lean forward and push into the wall, again trying to use your whole body.
Standing in a doorway, push outwards with your hands against the jambs. Place your hands low to work the shoulders, higher for the biceps and pectorals.http://www.sandowplus.co.uk/Competition/Hoffman/hoffmanindex.htmhttp://forum.dragondoor.com/training/message/378472/http://www.t-nation.com/findArticle.do?article=313iso2