As the first prop to win the players' award, Castrogiovanni finished ahead of Tigers team-mates Shane Jennings and Martin Corry, plus Bristol's Roy Winters and Saracens fly-half Glenn Jackson.
"I am very honoured, even to be in the list of names for the award," said Castrogiovanni, who joined Leicester from Calvisano last summer. "It was a big change for me coming from Italy to England but I am enjoying it and it has been a good year.
"Leicester have some top-class props in the squad and working with them and the coaches helps you improve your game so much."
Phil Winstanley, rugby manager of Premier Rugby, was among the judging panel and said: "In his first season in the Guinness Premiership, Martin has been a rock for Leicester Tigers in what may yet be the most successful season in the club's history. His enormous contribution was recognised by the panel in a position in the front row that isn't widely recognised".
Howard topped a shortlist that featured Alan Gaffney of Saracens, Bristol's Richard Hill and Dean Ryan of Gloucester.
"I am sure all the directors of rugby would say you are only as good as your players, and that goes for not just the frontline guys, but all of the guys who step up during the season," said Howard.
"It is an honour I will accept on behalf of everyone on and off the pitch at Tigers who has worked so hard all season and especially to the players who deserve a lot of credit."
Castrogiovanni, scrum-half Harry Ellis and hooker George Chuter were named in the Dream Team of the Season, chosen by Sky Sports duo Dewi Morris and Stuart Barnes, and unveiled at the awards ceremony which featured representatives of each of the Guinness Premiership clubs, as well as Premier Rugby's commercial partners.
Leicester wing Tom Varndell was nominated in the Try of the Season category, and Tigers were also shortlisted for the Performance of the Season for the opening-day win over defending Premiership champions Sale Sharks.
The full list of nominees and winners:
Guinness Premiership Player of the Season 2006/07
Winner: Martin Castrogiovanni, Leicester Tigers
Runner-up: Shane Jennings, Leicester Tigers
Third: Roy Winters, Bristol Rugby
Also nominated: Martin Corry, Leicester Tigers
Glen Jackson, Saracens
Land Rover Discovery of the Season 2006/07
Winner: Tom Rees, London Wasps
Runner up: James Haskell, London Wasps
Third: Dominic Waldouck, London Wasps
Toby Flood, Newcastle Falcons
Ryan Lamb, Gloucester Rugby
O2 Director of Rugby of the Season 2006/07
Winner: Pat Howard, Leicester Tigers
Alan Gaffney, Saracens
Richard Hill, Bristol Rugby
Dean Ryan, Gloucester Rugby
MBNA Try of the Season 2006/07
Winner: Mark Foster, Gloucester Rugby v NEC Harlequins, 17th March
Runner-up: Kieran Roche, London Irish v Bristol Rugby, 16th September
Third: Nick Abendanon, Bath Rugby v Leicester Tigers, 9th September
Tom May, Newcastle Falcons v Sale Sharks, 26th December
Ben Cohen, Northampton Saints v NEC Harlequins, 17th February
Tom Varndell, Leicester Tigers v London Irish, 18th November
Gilbert Golden Boot Award 2006/07
Winner: Glen Jackson, Saracens
Olly Barkley, Bath Rugby
Adrian Jarvis, NEC Harlequins
EDF Energy Community Player of the Season 2006/07
Winner: Matt Perry, Bath Rugby
Richard Haughton, Saracens
Dale Rasmussen, Worcester Warriors
Dave Walder, London Wasps
Gillette Power Award 2006/07
Winner: Alex Brown, Gloucester Rugby
Mark Cueto, Sale Sharks
Bruce Reihana, Northampton saints
Roy Winters, Bristol Rugby
The Times Team Performance of the Season 2006/07
Winner: Bristol Rugby (v Gloucester Rugby, 24 Nov, 14-12)
Leicester Tigers for Leicester Tigers v Sale Sharks (3 Sept) 35-23
London Wasps for London Wasps v Saracens (8 Apr) 27-26
Sky Sports Dream Team 2006/07
1. Martin Castrogiovanni, Leicester Tigers
2. George Chuter, Leicester Tigers
3. Darren Crompton, Bristol Rugby
4. Roy Winters, Bristol Rugby
5. Marco Bortolami, Gloucester Rugby (captain)
6. Juan Martin Fernandez Lobbe, Sale Sharks
7. Andy Hazell, Gloucester Rugby
8. Dan Ward-Smith, Bristol Rugby
9. Harry Ellis, Leicester Tigers
10. Glen Jackson, Saracens
11. David Lemi, Bristol Rugby
12. Toby Flood, Newcastle Falcons
13. Seilala Mapusua, London Irish
14. David Strettle, NEC Harlequins
15. Mike Brown, NEC Harlequins