Building a web-based tool on your own time and then letting your employer use it can potentially raise several legal and ethical issues. It is important to consider factors such as intellectual property rights, employment agreements, confidentiality, conflicts of interest, and potential liability. While I can provide you with a general overview, it is crucial to consult with a legal professional to fully understand the specific implications in your jurisdiction and industry. 

1. Intellectual Property Rights: When you create a web-based tool on your own time, there may be questions about who owns the intellectual property rights to the tool. In many jurisdictions, if the tool is created using company resources or during work hours, it may be considered the property of your employer. However, if you develop it entirely on your own time and without using any company resources, you may have a stronger claim to the intellectual property. It is important to review your employment agreement, any non-disclosure or intellectual property clauses, and seek legal advice to understand the ownership rights. 

2. Employment Agreement: Your employment agreement may have specific clauses regarding ownership of intellectual property, inventions, or developments created during your employment. Review your employment contract to determine if there are any limitations or restrictions on creating and using tools outside of work hours. 

3. Confidentiality: If your web-based tool involves any confidential or proprietary information from your employer, there may be concerns about breach of confidentiality. Ensure that you do not use or disclose any confidential information while developing the tool. It is advisable to consult your employment agreement or speak with your employer's legal department to understand the boundaries of confidentiality. 

4. Conflict of Interest: Developing a tool for your employer on your own time could potentially create a conflict of interest. It is essential to disclose your involvement in the development of the tool to your employer and obtain their consent to avoid any conflicts or allegations of impropriety. 

5. Liability: If the tool malfunctions or causes any harm, there may be questions about liability. It is important to consider potential risks and take necessary measures to ensure the tool is secure, reliable, and compliant with relevant laws and regulations. Consulting legal professionals can help you understand the potential liabilities and mitigate risks. 

To ensure a clear understanding and avoid any legal or ethical issues, it is strongly recommended that you: 
- Review your employment agreement and any related contracts or policies. 
- Consult with an attorney specializing in intellectual property and employment law. - Discuss your intentions with your employer and seek their written consent. 
- Ensure that you do not use any company resources or confidential information during the development process. 
- Comply with all applicable laws, regulations, and industry standards. 

Remember, the specific legal requirements and implications may vary based on your jurisdiction, industry, and the details of your employment agreement. Seek professional legal advice to obtain accurate and tailored guidance for your situation.