Hardy Boys Cartoon
In 1969, ABC began a cartoon series that would spawn a real-life rock band, new HB comics, and much more. Although the cartoons are comparable to other animation of the period, and Chet's voice is like eternal hell, I have a special little niche in my Hardy-heart for these animated atrocities.
Think late '60s and characters 'talking' by moving their mouths up and down... Interestingly, the Hardys and their friends formed their own band (sort of like The Archies, really) in the show called The Hardy Boys Plus Three, and there was also a real Hardy Boys band -- the real Dal McKennon, the voice of Joe Hardy and Chubby Mortonband members remarkably resembled their respective cartoon character! The band released two albums which produced three singles, an 8-Track, a cassette tape (!), and a reel-to-reel...thingy.
The Hardy Boys cartoon was the first to feature a black character, Pete Jones, in its adventures. It was also the first to have a thirty-second public service announcement at the end of the show.
All of the adventures were based (loosely) on a Hardy Boys book. Unnamed and unnumbered, it's hard to pinpoint the exact number of episodes, but Hardy Boys researchers have successfully compiled a thought-to-be-complete list. Each episode (thirty minutes) contained two different adventures, and a song from the HB band was featured in-between. I have been able to map all the performed songs to the released records except one (see below). A few of these episodes are available on VHS from toontracker.com and private collectors.
See: Characters  -  Cast  -  The Band  -  Episode List  -  Collectibles 

1969-71 - Characters
Frank Hardy
Joe Hardy
"Chubby" Morton
Pete Jones
Wanda Kay

[Voice] Cast
Byron Kane - Frank Hardy, Pete Jones, Fenton Hardy
Dal McKennon - Joe Hardy, Chubby Morton
Jane Webb - Wanda Kay Breckenridge, Aunt Gertrude

The Band
L-R: Reed Kailing, Bob Crowder, Deven English, Jeff Taylor, Norbert Soltysiak
L-R Character Names: Frank Hardy, Pete Jones, Wanda Kay, Joe Hardy, Chubby Morton
Information sent out from the Hardy Boys fan club included information on the five band members and a 45 record with the members introducing themselves. Several members went on to more popular groups (Reed Kailing was a member of Grass Roots and The Destinations).

Episode Guide
Each half-hour episode contained two 15-minute stories. Between stories (sometimes a little bit into the second one) the "Hardy Boys Plus Three" entertained viewers with a new song from one of their records. 
These episodes aren't in any kind of order. I've included the song in each episode, when known. Although the episodes don't include a title (making it very tough to figure out anything about them), titles have been assigned.

1. Footprints Under the Window/Hidden Gold -- Love & Let Love
2. Desert Giant/Viking Symbol -- Feels So Good
3. Old Mill/Missing Chums -- That's That
4. Cabin Island/Hidden Harbor -- My Little Sweetpea
5. Secret of the Caves/Figure in Hiding -- unknown
6. Secret Warning/Aztec Warrior -- unknown
7. Skeleton Rock/Chinese Junk -- unknown
8. What Happened at Midnight/Clue in the Embers -- unknown
9. Screeching Owl/Yellow Feather -- unknown

10. Devil's Paw/Haunted Fort -- Carnival Time
11. Sinister Signpost/Melted Coins -- unknown
12. Hooded Hawk/Short Wave -- Hello Girl***
13. Shore Road/Crooked Arrow - One Time in a Million
14. Phantom Freighter/Pirate's Hill -- unknown
These are episodes produced, but I
don't know how they pair up (all 1970):
House on the Cliff
Spiral Bridge
Mark on the Door
Flickering Torch
Wildcat Swamp
Broken Blade
*** This song was first released by Reed Kailing (aka Frank Hardy) and The Destinations, his former personal band. Although the Hardy Boys recorded a version for this cartoon, it doesn't appear on any of their recordings.

Collectibles From These Serials
1969/70: Hardy Boys albums - RCA
Here Come the Hardy Boys and Wheels
1969: Here Come the Hardy Boys reel to reel tape.
1969: Here Come the Hardy Boys 8-track
1970: Two of the three HB singles. (Sink or Swim/Love and Let Love is one single)
1969: Single with sleeve (Spain release)
1970 - Wheels and Love & Let Love sheet music.
1970 & 1971 - Gold Key Comics
Three (confirmed) and probably four of the comics were published in the Spanish language and distributed in Spain and Mexico. Unfortunately, it is muy difícil to find these Spanish-language editions in anything remotely resembling good condition, not to mention finding them at all!
Three Hardy Boys comics in Spanish.
1969 - Milton Bradley Board Game
1969 - Collegeville Costumes; Wanda Kay, Frank, and Joe.
1969 Viewmaster Reels - "The Mystery of the Caves"
Two envelope styles (both seen above).
1969 - Rolls Royce - Corgi Toys
(This is the car the band uses in the cartoons.)
Quite a rare item and generally brings $100-$275 or more!