Commercial units:

http://www.westmountainradio.com

http://buxcomm.com

http://home.att.net/~n8st/DDI-index.html

Free Diagrams:

http://www.mymorninglight.org/ham/PSKbox.htm

http://blog.marxy.org/2008/02/built-better-psk31-interface-box.html

http://kh2d.net/ham/soundcard.cfm

Ok, that should handle either commercial or roll your own interface boxes.   I will list some of the digital modes you can use now.  Will try to list at least one software for each, however some software packages such as MIXW will do several.

RTTY:  http://en.wikipedia.org/wiki/Radioteletype  http://www.aa5au.com/rtty.html

RTTY SOFTWARE:  http://mmhamsoft.amateur-radio.ca/mmtty/

CW:  http://www.arrl.org/FandES/ead/learncw/

ASCII:  http://en.wikipedia.org/wiki/ASCII 

AMTOR "A, B":   http://en.wikipedia.org/wiki/AMTOR  http://f6cte.free.fr/

PACKET 300 BAUD:    http://en.wikipedia.org/wiki/Packet http://members.fortunecity.com/xe1bef/packetsoftware.htm

SSTV:  http://en.wikipedia.org/wiki/Slow-scan_television http://members.fortunecity.com/xe1bef/sstvsoftware.htm

PSK31:  http://en.wikipedia.org/wiki/PSK31  http://www.digipan.net/

SITOR "A, B":  http://en.wikipedia.org/wiki/SITOR  http://www.tigertronics.com/sl_soft.htm

MFSK 16:  http://www.qsl.net/zl1bpu/MFSK/  http://www.xyz.net/~joe/software.html

WEFAX:  http://www.hffax.de/html/hauptteil_beginners_guide2.html  http://www.hffax.de/html/hauptteil_jvcomm.htm

PACTOR "1, 2, 3":  http://en.wikipedia.org/wiki/PACTOR  http://members.fortunecity.com/xe1bef/pactor.htm  http://www.airmail2000.com/

APRS:  http://en.wikipedia.org/wiki/Automatic_Position_Reporting_System  http://www.ui-view.org/

G-TOR:  http://www.arrl.org/FandES/field/regulations/techchar/G-TOR.html  http://kantronics.com/products/kamxl.html

CLOVER:  http://www.halcomm.com/docs/dxpreview.PDF  http://www.halcomm.com/AmateurProducts.htm

HELLSCHREIBER:  http://en.wikipedia.org/wiki/Hellschreiber  http://xoomer.alice.it/aporcino/Hell/index.htm

MT63:  http://www.navymars.org/central/reg4/al/MT63.htm  http://xoomer.alice.it/aporcino/MT63/mt63setup.zip

THROB:  http://www.wb8nut.com/digital.html#throb

These are but a very few links, however it is enough to get you started.  Remember to Google or use your favorite search engine.  Most software is free, but some is reasonably priced.  I personally like the multimode software, so I don't have to learn each one individually.

There used to be more multimode TNC's back in the late 1980's-1990's.  Please be careful as some early models didn't have enough memory and / or can't be upgraded.  I personally like the Kam Plus Version 8.0 myself when you can find one. 

Now for the Multimode TNC's:

Kantronics KAM XL: http://kantronics.com/products/kamxl.html

Timewave PK-232 DSP:  http://www.timewave.com/support/PK-232/PK232PSK.html

HAL:  http://www.halcomm.com/AmateurProducts.htm

I hope this helps you get started into the wonderful world of HF Digital Communications.

73

Kenny

ARS KG5KS