There is a fairly extensive list at DF4OR's page. The trouble with most of those is that they won't work with a laptop whose serial port provides only about 5V. I also worry about blowing the handheld with some of them.
So I put together my own. I happened to have a couple of Maxim 639 chips, and I used one to get 5V supply from a 9V battery with minimal loss. Of course one could also use an efficient low dropout 5V regulator instead, such as the TPS 7250. I also used an optocoupler to isolate the PC output from the ICOM. Here is the cicuit I came up with.
The Maxim chip specs say that D1 should be a Schottky diode, but that an ordinary rectifier should be OK with small loads. I used the standard 1N4148 with no apparent problems. The inductor has to be chosen carefully, otherwise the output is a triangular ramp going up and down by a significant fraction of 1V. I ended up connecting in series three small inductors I had lying around, for about 200 uH total. The grounds of D1, the inductor and the capacitors should be as close to each other as possible. The circuit seems to be drawing just a few microamps when idle, but I did add a switch to the 9V battery connection.
The level converter itself uses a standard transistor output optocoupler, e.g. an MCT2. On a typical 6-pin optocoupler the LED is on pins 1-2 and the collector-emitter of the phototransistor are on pins 5-4. D2 is again a 1N4148, and the transistor is any standard NPN such as the 2N3904.
The ICOM connection is via a stereo cable cannibalized from old headphones, with the ground connected to the body of the 3.5 mm plug, and the output/input connected to the middle section of the plug (the very tip is used in the T90A for the speaker/earphone).
The PC connection is via a cable cut off from an old mouse, with a female DB9 plug at the end. Pin 3 (TD) goes to the optocoupler, pin 5 is signal ground, and pin 2 (RD) goes to the collector of the transistor. I also had to short the wires going to pins 6, 7 and 8, or else most programs didn't want to open the serial port at all.