www.etherdecode.com

Can't I do all this with Wireshark ?
To some extent (and more), but the point of Etherdecode is that you can see ethernet timing relative to other signals. You may also prefer a scope type user interface

Will it work with those really cheap 8-channel USB logic analysers ?
Yes, but probably not very well. If you only use it to generate an ethernet trigger then it may be quite useful, however as the decoding functionality uses  all 8 channels, it doesn't allow viewing of ethernet packets relative to other signals so Wireshark may be a better solution.
Secondly, these cheap USB analysers typically sample at 24MHz, so the 12.5MHz output rate of Etherdecode will make the bus data look rather uneven as the sample rate is marginal.  See this  Screenshot - you can see that every 11th or 12th sample is shorter

If I just want ethernet triggering, can't I just connect a scope to the ethernet signals?
No, because of the complex encoding used by 100Mbit Ethernet, it is not possible to distinguish between idle and when a packet is being sent by scoping the signals on the cable. scope screenshot

Why not use SPI or UART instead of parallel bus?
The parallel data rate ia 12.5MHz, so a serial format would need to output at 100MHz, and no scope can do decoding that fast.

What is the minimum useable spec for MSO/logic analyser ?
At least 8 digital channels ( 3.3v logic levels), minimum recommended sampling frequency 25MHz. Ability to display 8 channels as a single bus.