OK, so they are not all technically accessories, just things I find myself buying over and over…
If anyone else has suggestions, please feel free to drop me a line at ferret701@yahoo.com.  Also, they are in no particular order.  

1. Wargame Accessories Metal Bases.  Wargame Accessories makes steel bases available in a variety of sizes.  Perfect for mounting infantry, vehicle and guns.  Fairly cheap (typically $5 for 50 of the smaller sizes).  They’re steel, so they’ll stick to magnetic paper in storage cases. 

2.  CinC .50 Caliber And 14.5mm HMG.  Sold in packs of 40 (Soviet) or 50 (US) guns on CinC’s site for about $5, they extremely well-molded models are great for adding a touch of realism to models, as many kits don’t come with them.  Also great for replacing those that get broken off. 

3. GHQ WWII Seated US Armored Infantry.  A fine collection of figures from GHQ, suitable for most eras and nations (unless you are way more picky about realism than I am).  Includes a number of standing figures, some looking like commanders, others manning heavy machine guns.  Sitting figures comes in groups of various sizes, but can easily be cut into smaller groups or individuals.  They have many potential uses, and have one big advantage – they are far more to scale than the GHQ infantry, so they fit well in vehicles and around artillery pieces.

4. GHQ WWII Halftrack Accessories.  An excellent college of baggage, jerry cans, tarps, etc from GHQ.  Useful for customizing vehicles, although not all models are suitable for modern tracks. 

5. Panzer Keepers.  The ubiquitous tank carrier from I-95 Enterprises.  Pretty much everyone makes them, but I-95 is generally your least expensive option.  Each holds 20 tanks or 40 smaller vehicles (think Scorpion-sized or smaller).  In addition to the basic size, larger versions equivalent to GHQ’s Bunker Box are also available.  Sometimes available in bulk fairly cheaply from I-95 on eBay.

6. GHQ/CinC M113s.  Probably the most ubiquitous armored vehicle in the world.  Most any army with any links to the West used them, usually in large numbers.  GHQ makes a better looking model of the M113A1, but CinC has a good collection of different vehicles that are both cheaper and probably more accurately to scale.  You’re going to need a lot of them…  (actually, the split between the companies works well here – GHQ makes the M113A1 and M113A3, while CinC has the M113 slick and the M113A2).  Oh, and its only referred to as the "Gavin" but nut jobs.

7. GHQ WWII Soviet and US Artillery Crewman.  Excellent figures, well scaled to go with any manufacturer’s guns.  Yes, they are for WWII – but the differences are absolutely minimal at this scale.  Most NATO armies had helmets like the US did in WWII for most of the Cold War, as did the US for a pretty good chunk of it.  As far as I can tell, the difference between the Soviet WWII uniforms and the Cold War variety was minimal enough not to be noticed at this scale.  Besides, every artillery piece looks better with a crew.

8. LocTite QuickTite Super Glue.  LocTite makes super glue in a large, hard plastic container with squeezable sides and a screw on cap.  Great for controlling the amount you get, and for getting the most out of the container.  Not cheap at $3 or so a pack, but works great.  Plus it’s a gel-style super glue, so it doesn’t run all over the place.  Available at any hardware store, and probably most Targets and Wal-Marts.

9. Woodland Scenics Blended Turf.  This stuff comes in a vast array of colors and textures, and I probably own most of them, but you can probably get by with just 2 to start with:  Blended Turf Green and Blended Turf Earth.  They are the basics of any flocking you’re going to do to your bases, and by mixing them, you can produce a variety of shades.  Don’t be afraid to throw in a bit of sand box sand – a little variety helps. 

10.  Sharpie Ultra Fine Markers.  The classic Sharpie permanent marker, but with an ultra fine tip.  Comes in a variety of colors, generally in one pack (such as this one) for under $10.  There are a wide variety of uses – marking the bottom of vehicles or stands for gaming, detailing (great for red tail lights), etc.  Don’t like the way it looks?  Just paint over it.  Dries very fast and on any surface, though it may take a few seconds.  These markers have one advantage over many other things on this list – one set should last you many, many years.