PALM ICON EDITOR
Who wants to blunder around the desktop making icons with a mouse when you can draw with carefree abandon and a stylus on a handheld? If you have a Palm - or a Treo - you can make your agenda icons directly on your pda or phone.
The simple icon editor DbIcon+ comes free with the Datebk Agenda application for Palm
Here's a screen shot of the dbcon+ editor

I made this quick apple for my agenda category 'Health'.
At least it shows up well.
I was able to choose from a wide selection of colours. I could also have called up a slider with RGB colours if I had preferred.
I made these illustrations a while ago in the DateBk5 Icon Editor. Datebk has since scaled up to Version 6 (DateBk6). The old icon editor is still supplied, but Pimlico Software, the developer, now recommends that you switch to Icon Manager.
Icon Manager is free but more powerful. I don't at the moment use a Palm, but last time I tried it, Icon Manager had high and low res drawing pads with useful image controls. Besides shifting, rotating, mirroring and otherwise playing acrobatics with a square selection, I could adjust the brightness and contrast to make an icon stand out. Cutouts were catered for with transparency.
COLOUR CODED TEXT
Ah! Now I can no longer use DateBk5, do you know what most I miss? The ability to colour code TEXT. Fonts for categories can be colour coded in Datebk. Not only can you colour the lettering, but also the strip of background, to highlight the category of a task or event.
I'm amazed that this simplest of means to differentiate entries is ignored by so many other agenda applications.
Neither my HTC TyTn (a Pocket PC) or Sony Ericsson P990 (Symbian UIQ3) has seen fit to oblige me with category-coloured agenda text. I can only colour code calendars on my desktop Mac. (Calendars sync - via The Missing Sync - as Categories from iCal for Mac.)
In DateBk, to match your highlighted strips of text, you can even forego multicoloured icons, and make them appear in monochrome. That is, they'll use only the colour assigned to the lettering of the category they represent. In this mode it's even easier to pick out different types of activity. (As long as your icons have easily differentiated shapes to make up for the single colour.)
You can even do without category icons altogether in text-heavy views. Hrrrmph! I shouldn't be saying this on a page about making icons, but Category icons are not indispensable when you have a coloured strip to highlight the type of entry.
Status icons then stand out more clearly.
This unclutters the screen, and leaves the category symbols for their best use - instead of, rather than as well as, lines of text.
Below is a clip from the month view in Datebk5 with the little apple in use in multicolour mode. The screen grab shows on the desktop about one and a half times larger than it does on the Palm, but the icons are still pretty small. Not much room for text there!
Where there isn't room for print, that's when the tiny symbols come into their own.

Agenda icon making, free and fun on Palm and Treo