There are basically two reasons for training in another martial art. First is to find compatible training that will enhance the skills that you are developing in the Aikido. From that standpoint many people find that Iaido complements their Aikido training well. It is basically a form of moving meditation and will teach you the basics of handling a blade properly (some debate here). If you are lucky enough to have access to a qualified teacher of Classical Sword (Caveat Emptor on the various claims to teaching credentials) it would be invaluable. Jodo is more available than sword in terms of classical styles. Chuck Clark sensei could tell you what he thinks about the benefits. An excellent source for info about where to train would be Phil Relnick Sensei (see http://www.shindo-muso-ryu.org/?prelnickbio.html
for information about this top notch teacher) It will certainly improve your Aikido from a number of standpoints. I know a number of Aikido instructors who have chosen to go cross-cultural and study T'ai Chi Ch'uan. Uniformly their Aikido technique tends to be relaxed and beautiful.
The second reason to do another martial art is to improve your capacity to do your Aikido effectively from a martial standpoint. There are many avenues you can take here. Bruce Bookman Sensei took a couple years of boxing instruction under a coach and has now spent quite a bit of time working on Machado Brazilian Jiu Jutsu ( http://www.machadojj.com/
) . Can't think of too many people I'd rather have with me in a tough spot.
If I were to take on another art there are essentially two areas on which I'd focus. First, I'd look for a school that had a qualified teacher of Jeet Kun Do. Their training will give you an eclectic mix of Muy Thai, Wing Chun derived technique, and Filipino martial arts. The Wing Chun components will teach you trapping and stripping. Aikido people tend to rely a lot on grabbing an opponent and this training will cure you of that fast. Also, the Filipino components will teach you how people in the real world use bladed weapons (usually a shock to folks who have only done Aikido). The flow drills that are taught in Filipino martial arts are wonderful for teaching Aikido folks to use both hands separately and handle combination attacks, which is a weak area for most Aikido folks. (See the wonderful instructional videos by Robert Bryner Sensei for an example of how you can incorporate some of the Filipino exercises in a traditional Aikido practice - http://www.aikido-world.com/highlights/bv/videos.htm).
The style of Jeet Kun Do that has the most interest for me is done by Sifu Larry Hartsell, one of Bruce Lee's original students ( http://www.jkdassoc.com
) . He was interested in grappling and is considered to be the source for Jeet Kun Do grappling application. His style shows you how a lot of the locking technique that we do can be done against more conventional styles of fighting.
If you want a real humbling but educational experience take a look at Mr. Vasiliev up in Toronto, Canada ( http://www.russianmartialart.com
) . I would dearly love to get to train with this man, he is clearly a master of unique skill. He is a former instructor of what the Russians refer to as the "Systema". It was only taught to the Russian Special Ops folks like Stalin's body guards and the Spetsnaz commandos. Before you prejudge this stuff you should take a look. It was totally different from what I had expected when I ordered some of his tapes (every one is worth twice what they ask). The Systema is aikido without any of the form. Every thing is done with no set postures, even designed to be done from positions of great disadvantage. Their training has no set form either. It is basically a form of jyu waza and randori which gets faster as you progress. I have been working off some of Mr. Vasiliev's tapes and they can help you immensely with your Aikido I believe. The level of his relaxation is an inspiration yet he is quite effective. This stuff is very compatible with Aikido. And Mr. Vasiliev is a real gentleman. I had occasion to correspond with him and he was extremely gracious. The only problem here is that there are virtually no qualified instructors around who teach this. There is an American teacher on the East coast who teaches some of the same stuff but is with a different organization (ROSS Russian Martial Arts) but I have only seen his website ( http://www.amerross.com/main.html
[Edited by George S. Ledyard on July 29, 2000 at 04:51am]