Well.. I was going to say Truxton but if that's already taken how about:

Stinger (NES) - Very easy shooter with colorful visuals.
Retro Game Challenge (DS) - Has two shooters that both lean to the easy side.
Blazing Lasers (TG16) - A lot like Truxton.

axelay for snes
sidewinder for nes
river raid for atari 2600
pop n twin bee for snes
mega man 2 for nes
zero gunner for arcade

Try out Firepower 2000 (overhead) and Super E.D.F. (sidescrolling) on SNES for some rather forgiving scrolling shooters. Neither are available on other platforms but may be available on UK computers like the Amiga (yes, it was released in the US but it was more popular in the UK by far).

Thanks so far! I'm certainly gonna give the NES and SNES recommendations a try. Axelay sound very familiar. Gave to do a youtube search on that

Any Genesis suggestions?

without having played it. a few people recommend lightning force / thunderforce iv for genesis, though it may be difficult. also i can't remember if pop n twin bee was for snes or ps1, or if both which version i played.

Thunder Force IV/Lightening Force is a lot harder than anything mentioned so far in this thread. If you want a Thunder Force game, I would suggest going with Thunder Force III, it allows selectable starting planets so you can get a "Mega Man" feel with it.

Task Force Harrier EX on Genesis is a harder to find, earlier, title but it is pretty forgiving from what I remember.

Some to avoid on Genesis, though they are awesome after you gain some skills so don't pass up if cheap:

Gaiares (the bosses are AWESOME)
Atomic Robo Kid (this one seemed to control like mud causing more headache than good but others love it)
Wings of Wor (if you want to control a flying winged man and don't own a Turbo Duo...)

Contra III (SNES) has an easy mode.

Thunderforce 4 / Lightning Force has a cheat code for unlimited lives. Also, I second anything by Compile (Blazing Lazers, MUSHA, Space Megaforce, etc). Galactic Attack (aka Layer Section) is well balanced and playable. The Silpheed games are easy to come by and playable. Raiden Project for the PSOne is good too. Aero Blasters on the TG 16 or Sega Genesis comes to mind. Wings of War (GYNOUG if you're in Europe) is good too. Short though.

If import games are OK, try "Bokan to Ippatsu Doronboo Kanpekiban" is a Sega Saturn cute 'em up based on an old cartoon called Time Bokan. It's about as easy as a Shmup gets and would be good if you got kids you want to introduce to the genre. No Japanese required, but you'll need an Action Replay to play it. 100% Cotton is good too, but I'd steer clear of the Saturn Cotton games, they're pretty tough. If you want to try something bullet hell but w/o the impossibly hard gameplay try Dodonpachi. It's about as light as bullet hell gets, and completely playable by normal people .

Lastly, if you're OK with neo retro Jamestown is a bullet hell game with variable difficulty. You can get it right now in the Humble Indie Bundle (I paid $6 bucks for it and a bunch of other games). On the normal difficulty level it hardly qualifies as bullet hell, and if it wasn't for the gameplay elements it cribs from Dodonpachi I wouldn't recognize it as such.

Ikaruga, an indie game on the 360 called Decimation X 3 ... and ... Millipede on the 2600. kthx